The present invention relates to the general field of turbine engine blades made of composite material comprising fiber reinforcement densified by a matrix.
The intended field is that of rotor blades for assembling on gas turbine rotor disks for aeroengines or industrial turbines.
This type of blade was originally made by casting and included a root in the shape of a bulb. The as-cast root was subjected to precision machining in order to provide an effective mechanical interface with its housing in the rotor disk.
Proposals have already been made to fabricate similar turbine engine blades out of composite materials. By way of example, reference may be made to patent application US 2011/311368, which describes fabricating a turbine engine blade by making an airfoil preform by three-dimensional or multilayer weaving and then by densifying the preform with a matrix. The root of the composite material blade reproduces the bulb shape so as to take up centrifugal force and facilitate incorporating composite material blades in an existing engine environment.
For blades made of composite material, the blade root is made by using an insert that is positioned in a region of non-interlinking in the textile preform so as to form a bulb-shaped portion in that part of the blade that corresponds to its root.
Nevertheless, that technique of forming a blade root makes industrial fabrication of the blade more complex and increases its fabrication cost, since it leads to considerable losses of material and requires difficult handling that slows down the speed of production. Furthermore, the insert which is also made of composite material needs to be densified and machined, thereby leading to additional costs and possibly to parts being rejected.
The textile of the preform, which is naturally floppy, interacts mechanically with the insert and can lead in particular to textile shear, to the insert turning, to interlinking being lost between the insert and the textile, etc.
Furthermore, molding and densifying the portion of the preform that is to form the blade root are found to be difficult, in particular because the tolerances on the profile of the bulb-shaped root are very small (of the order of one-tenth of a millimeter) and because requirements in terms of mechanical properties for this portion of the blade are significant, since the blade root concentrates the majority of the forces that are applied to the blade.
Document US 2010/189562 discloses a turbine engine blade made of composite material having a substantially plane portion in its part that is to form the blade root, the root shape being obtained by clamping this portion between two metal plates that are held in place by a welded stud. That design makes it possible to facilitate fabricating the blade out of composite material, since the root geometry of bulb or equivalent shape that is difficult to obtain from the textile preform is provided by adding metal plates against the flanks of a plane portion, which is simple to make out of composite material.
Nevertheless, as mentioned above, the root of a blade corresponds to the portion of the blade that concentrates most of the forces applied to the blade, since it serves to hold the blade in the disk against centrifugal forces. When the metal plates are held by a welded stud, as described in Document US 2010/189562, the forces applied by centrifugal force are taken up essentially via that portion of the composite material of the blade which is in contact with the stud, whenever the friction forces between the metal plates and the flanks of the blade made of composite material are not sufficient for taking up those forces. That situation therefore leads to a risk of the composite material being damaged, or indeed of it being ruptured or crushed.